1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive recursively estimating repeatable runout (RRO).
2. Description of the Prior Art
Repeatable runout (RRO) in a disk drive is a disturbance in the servo system which can degrade performance by disrupting the head's centerline tracking during read and write operations. The RRO disturbance is typically caused by physical imperfections in the disk drive such as spindle motor runout, disk slippage, disk warping, media defects, and imperfections in the electromechanical servoing mechanism including the mechanism for writing embedded servo sectors onto the disk during manufacturing. Because the imperfections that cause RRO are relatively static, RRO is a predictable disturbance that is periodic with the rotation of the disk. It is known in the industry to estimate and cancel out the periodic RRO disturbance by introducing a feed-forward compensation signal into the servo loop.
Prior art techniques typically estimate the RRO disturbance by averaging the position error signal (PES) over many revolutions of the disk in order to average out other noise sources (the non-repeatable runout), and then backing out the effect of the servo compensator. The RRO disturbance is estimated during a manufacturing process, wherein a feed-forward compensation value is generated and stored for each servo sector of each track. However, estimating the RRO disturbance for every track over multiple revolutions of the disk can increase significantly the manufacturing time of the disk drive.
There is, therefore, a need for a fast, accurate technique for estimating the RRO disturbance in a disk drive to reduce the manufacturing time.